The vast majority of heat pumps and refrigeration systems today employ variations on the vapor-compression thermodynamic cycle. This approach changes the pressure of an inert working fluid to manipulate its associated saturation temperature, which allows for the transfer of sensible heat across a metal surface as the fluid changes phase. This technology is mature and has been heavily developed, but there remain several limitations. For example, many of the most efficient refrigerants have undesirable effects on the atmosphere such as depletion of the ozone layer by chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) refrigerants. Although hydrofluorocarbon (HFC) refrigerants do not deplete the ozone layer, they have significant direct global warming potentials (GWP). Even though refrigerants are nominally contained within a closed system, current industry practices added the equivalent of 144.9 metric tons of CO2 to the atmosphere in 2007 through the release of these high Global Warming Potential (GWP) gases. Further, the use of sensible heat transfer across the walls which isolate the refrigerant tend to require sub-cooling below the saturation temperature of the moist air to decrease its water content through condensation, a requirement that limits the possible thermodynamic efficiency of the process.
Thus, there is a continuing need for improved air conditioning and refrigeration systems.